The present invention relates to an optical rotary encoder.
Conventional optical rotary encoders include a pulse scale type using a rotational disk formed with slits and a Doppler type using a rotational disk formed with a diffraction grating.
FIG. 15 illustrates an example of the arrangement of the pulse scale type rotary encoder. As shown in FIG. 15, the pulse scale type rotary encoder includes a disk 33 formed with slits 32 at equi-interval along its circumferential direction. The center of the disk 33 is attached to a rotational shaft 31. The rotary encoder further includes a light emitting element 34 and a light receiving element 35 disposed to confront with each other with the slits 32 intervened therebetween. Light emitted from the light emitting element 34 is received in the light receiving element 35 in a form of a pulse signal in synchronization with the intervals between the slits 32 and in association with the rotational speed of the shaft 31. The number of the pulses is counted to provide a rotational angle of the rotational shaft 31.
The Doppler type rotary encoder is proposed in, for example, Japanese Patent Application Publication (Unexamined) No. 61-66927, which discloses a rotary encoder comprising a radiation grating formed on the periphery of a rotational disk. It detects the rotational angle of the disk based on the amount of a Doppler shift or Dopper frequency of the diffraction light of a laser beam irradiated onto the radiation grating.
The above scale type rotary encoder needs smaller intervals between the slits 32 in order to improve the detection accuracy. However, light diffraction occurs if the intervals between the slits are excessively narrowed, with the result that the detection accuracy is adversely reduced. To avoid the diffraction, the disk 33 should have a larger diameter.
With the rotary encoder proposed in the aforementioned Japanese Patent Application Publication, it is necessary to for a laser beam to be reflected, deflected and synthesized in a complicated manner to irradiate the beam onto two points on a rotational disk in order to avoid errors due to unequal pitch between adjacent radiation gratings or due to a possible eccentric mounting of the rotational disk. Further, the rotational direction cannot be detected without using such a complicated system. Furthermore, a detection error may occur due to a variation in frequency of an oscillated light caused by a laser device per se.